Tooth Size Variation in Assemblages of Tremacyllus (Hegetotheriidae, Notoungulata): Insights into Geographical Gradients, Systematics, and Sexual Dimorphism

Abstract

Tooth size variation within fossil assemblages can be associated with intra- or interspecific variation, functional, developmental, and geographical factors, and/or sexual dimorphism. Understanding these sources of variation is necessary to develop diagnoses for fossil mammals, where teeth are usually the most frequent remains. Tremacyllus (Ameghino, 1891) (Hegetotheriidae, Notoungulata) is a genus of small-sized herbivorous mammals abundant in late Miocene to Pliocene outcrops of southern South America. Its simplified, euhypsodont dentition and size variability have hampered systematics studies and led, for instance, to an overestimation of the number of species. I analyzed tooth size variations within assemblages of Tremacyllus in a quantitative framework to test three hypotheses: (1) magnitudes of size variation are different among tooth loci and assemblages; (2) tooth size follows a geographical pattern within the analyzed sample (Bergmann’s rule), but is also associated with taxonomy; and (3) there is a correlation between size variation and sexual dimorphism reflected in distinguishable subgroups. Results indicate that patterns of variation might be associated with eruption time and/or functional position. Northwestern forms are larger than southwestern-Pampean ones, not conforming to Bergmann’s rule but revealing a strong influence of latitude. Size differences between assemblages agree with dental features that distinguish T. incipiens and T. impressus, allowing expanded species diagnoses. Two size subgroups might reflect sexual dimorphism in the absence of biostratigraphic or morphological differences between them. This interpretation indicates that northwestern specimens referred to T. diminutus should be referred to T. incipiens.